This invention relates to methods of making optical waveguides, optical waveguide preforms and other optical products requiring high quality by vapor deposition techniques, and more particularly to an improved method to dry and consolidate porous waveguide preforms of vitreous particles in order to lower residual water content and entrapped gases.
The most commonly used techniques for low attenuation optical waveguide manufacture at present are based on dissociation in a flame of glass forming constituents, i.e. flame hydrolysis, to build up a porous waveguide preform of vitreous particles called "soot". The porous soot preform is converted to a glassy state by drying and consolidation at elevated temperatures. The desired combination of core and cladding layers is drawn under temperature and tension to the desired final diameter of the optical waveguide or fiber. As the usage of optical waveguides has increased, the technical requirements for communications systems have also become more demanding. In the current state of the art, signal attenuation of 0.4 dB per kilometer, low dispersion, and precise cutoff wavelength characteristics are required. These factors dictate that, in addition to reliable and predictable control of the refractive index profile as defined by the core and cladding geometries and materials, there must also be very low impurities and a homogeneous microbubble-free structure in the optical waveguide.
One major cause of light attenuation by scattering or absorption is due to the presence of residual water or other entrapped gases within the glass material. Residual water means the glass contains either OH, H.sub.2 or H.sub.2 O, but in essence is the OH, or hydroxl ion that is present. It is necessary to restrict hydroxyl ion content to keep losses low, because OH ions, introduced at interfaces or during processing, are representative of H.sub.2 O content, which is directly proportional to absorption losses. In order to produce optical waveguides wherein attenuation is kept below 0.4 db per kilometer, residual water content within the glass must be reduced to low levels. In addition, microbubbles and foreign particles may also be entrapped within the glass material or at the core and cladding interface and thus become a source of light scattering centers.
Various procedures and processes have been utilized to reduce residual water content in flame hydrolysis produced porous waveguide preforms. In U.S. Pat. No. 3,868,170, there is disclosed a method of drying and consolidating a flame hydrolysis produced porous waveguide preform by placing it in a controlled environment in which a predetermined desired concentration of gases is maintained. The porous preform is then heated to a temperature below the sintering temperature of the glass to permit entrapped gas to escape therefrom. The temperature is maintained until an equilibrium is reached between the partial pressure of the entrapped gas in the preform and the partial pressure of the same gas in the environment. The preform is then heated to at least the sintering temperature of the glass in order to sinter the porous preform and form a consolidated preform.
U.S. Pat. No. 3,933,454 discloses a method for producing glass with low residual water by heating the soot preform to a temperature within the consolidation temperature range for a time sufficient to cause the soot particles to fuse and form a dense glass layer while simultaneously subjecting the soot preform to a stream of a substantially dry atmosphere containing chlorine. The chlorine permeates the porous soot preform during the consolidation thereof and replaces hydroxl ions with chlorine ions, thereby resulting in a glass that is substantially water-free. Other methods also teach the necessity of using chlorine as a drying and cleaning agent during both the drying and consolidation of the porous soot preform.
Because of the need for continually lowering the attenuation of optical waveguides, improvement in the methods of obtaining optical waveguides with low residual water content is required. Therefore, it is evident that a method of producing a waveguide preform which is substantially free of residual water and other entrapped gases from which may be formed, an optical waveguide that will not cause excessive light absorption losses or excessive dispersion of the transmitted light, and does not have light scattering centers within the optical waveguide core or the core/cladding interface is desired.